The use of ORIF (Open Reduction, Internal Fixation) has become a mainstay in the treatment of a variety of medical conditions. The corresponding design of orthopedic implants used to achieve this fixation has progressed through the development of plates or other stabilization means, including for example rods and or mesh, intended for application to specific bones, and for use with characteristic types of medical conditions. These plates are optimally designed to correspond to a generalized shape of the bone. The systems that use these plates include fixation means or fasteners, which are usually screws or pegs, that hold the plate to the bone, and further hold fragments of the bone to the plate in association with other fragments so that the fragments will fuse together.
In position on the bone, the plate, fasteners and bone form a “construct” that accepts dynamic loading. The interaction of the fasteners, which are typically screws or pegs, with the bone, and with the plate is a complex matter. Typically, screws include a threaded shaft and pegs may include a threaded shaft or may simply be cylindrical and be devoid of threads about the shaft. Bone has a hard cortical surface and a porous cancellous internal portion and the variable nature of bone must be taken into consideration in the design of an implant system. Further, as bone lives, it reacts to loading and to motion of the fasteners so that threads can loose purchase over time as the bone shifts away from the threads. Further loading between fragments influences fusion of the fragments. Thus, the design of the plate/fastener interface includes considerations of loading as well as accommodation of typical patterns of fragmentation and the intention to capture cortical surfaces and to avoid surrounding soft tissue.
The use of fixation plates generally falls into two categories, the repair of fractures and the reconstruction of degenerative bone pathology. Over time, the sophistication of the implants used to treat these conditions has increased dramatically. Initially, a fixation plate and its associated fasteners were not mechanically fixed to one another. In these early plate and fastener systems the trajectory of the fastener tended to be defined by the trajectory of the fastener hole with in the plate since this would be used to guide the fastener, or drill guide to form the fastener hole within the bone. In this type of fixation, the fasteners pass though the plate and into the bone. Fixation in this type of system is achieved by friction between the bone and the plate/fastener construct, and the construct is referred to as having “non-locking” fixation.
In contrast, fixation plates have developed in which the fasteners are mechanically locked at a predetermined and fixed angle relative to the plate, often through the use of a fastener having a threaded head which mates with internal threads in the fastener holes of the plate. This type of fixation is referred to as “locking fixation.” Locking fasteners have the advantage of being less likely to back out of the plate, or to provide proud surfaces that can result in irritation to surrounding soft tissue. Moreover, in locking plate systems, the angle of the fastener axis relative to the plate, and the length of the screw is determined to account for capture of bone fragments for typical injury or deformation. In some ways this makes the surgeon's job easier in deciding on fastener placement. Often in this mode of fixation, a drill guide is used in conjunction with the fastener hole to drill the fastener hole within the bone.
In the locking plate construct, fixation strength does not rely on the bone and plate interaction. This facilitates the creating of a “bridge” construct that was once only possible by using a large external frame. The majority of internal fixation plates used today are the locking variety. This type of plate is especially useful in patients with low density bone, often the elderly or diabetic, where the necessary screw purchase for a non-locking construct is not present. While these types of plates, known as “locking plates”, have become the standard of care in both reconstructive and trauma plating, there is evidence within the medical community, that there are some potential problems with overly stiff locking constructs. There is growing evidence that some locking constructs have become too stiff to allow for proper bone healing. There is evidence that non-union rates trend higher with extremely rigid stainless steel locking plates when compared to more flexible titanium plates. There have been published studies that suggest that a less rigid but strong construct may be optimal to provide stability while not interfering with the normal fracture healing mechanics and physiology.
“Variable angle” locking technology, often described as “poly-axial”, refers to the ability to choose the angle of the fastener axis relative to the fastener hole within the plate and to lock the fastener at that angle in the plate. This mode of fixation provides the surgeon with the ability to create a rigid construct while allowing the flexibility to place the fixation screws at the optimal trajectory. In this type of fixation, the angle is determined through the use of a drill guide, which sets the angle in the bone. Current solutions on the market do provide the ability to vary the angle of the locking screws, but they do so by significantly compromising the strength of the interaction between the plate and the screw. This creates a greatly weakened overall construct when compared to a traditional fixed angle locking construct. Additionally, the current variable angle locking designs modify both the plate and screw in every, non-compression, hole within the plate. This leaves all of the plate holes compromised even if the nominal angle of the screw hole would have been appropriate.
The present invention addresses the weaknesses of the systems currently on the market. It increases the strength of the variable angle locking mechanism so as to make it more clinically effective. Further, increasing the strength reduces the chance of construct failure and resultant non-union of the fixed bones. This has serious consequences for both the surgeon and the patient being treated. A stronger mechanism will also allow the technology to be deployed beyond the current scope of the prior art products due to this inherent relative weakness. Additionally, initial testing of the present variable locking mechanism demonstrates that the system provides fixation that is stronger but less rigid and the current literature suggests that this may provide a significantly positive impact on fracture union rates.
The variable locking mechanism of the present invention can be used with existing threaded locking plates without change to the plates or the addition of steps to the surgical procedure. The present invention enables three modes of fixation within a single threaded hole, and does not even require any modification to present locking plates or any significant modification to the current surgical procedure used to achieve fixed angle locking. This is accomplished by installing a locking ring into the plate with an installation tool that resembles a traditional locking screw drill guide. This ring will be installed such as by a friction fit on the drill guide, and at the surgeon's discretion the ring driver is used in place of the traditional drill guide. In a further embodiment, the drill guide has a conical opening and a scalloped edge. The ability to add function to an existing system without adding steps or altering the typical flow of the surgical procedure is a critical user need for almost any surgical product and greatly enhances the acceptance and ease with which a new product is adopted.
The present invention also provides a variable angle non-locking fastener, which can similarly be used in the threaded locking holes of existing plates. Fasteners are provided within the same system (and in particular in the same surgical tray), which have a rounded smooth head that is sized to ride on the threads of the fastener hole so as to provide for a variable angle non-locking relationship with the plate. The drill guide is used to set the angle in the bone, which in turn sets the angle relative to the plate. Thus, the present invention provides the surgeon with three modes of fixation (locking fixed angle, locking variable angle and non-locking variable angle) using virtually the same plate, instruments and surgical procedure that the surgeon has become used to for fixed angle locking fixation. The present invention provides the surgeon with the ability in his or her discretion to select the mode of fixation in the operating room as the needs of the patient dictate. Without any significant change to inventory (the plate inventory remains the same, and the change is simply the addition of two types of screws, the locking inserts, the locking insert driver, and a drill guide for the non-locking screws.) The present invention allows the surgeon to decide once he has had a chance to view the open surgical site whether to utilize the strength of a fixed angle lock at a pre-selected angle or to alter the trajectory of the fastener the strength of the relation between the plate and the fastener.
The present invention can be used in any number of surgical applications, including for example, for any orthopedic implant application such as for example for cranio facial plates, trauma plates, small bone plates, long bone plates, and for the spine or pelvis.